System for supplying power to an electric arc metallurgical furnace



Jan. 2, 1968 w. R. STU RROCK ET AL 3,361,862

SYSTEM FOR SUPPLYING POWER TO AN ELECTRIC ARC METALLURGICAL FURNACE Filed June 15, 1964 2 Sheets-Sheet 1 440 V. 3 PHASE INVENTORS WILLIAM R. STURROCK ALLEN J BAKER HENRY W. MC ROBBIE 59214146 'Arromvz Jan. 2, 1968 W. R. STURROCK ET AL SYSTEM FOR SUPPLYING POWER TO AN ELECTRIC F iled June 15, 1964 ARC METALLURG I CAL FURNACE 2 SheetsSheet 2 POWER SOU RCE A.C. POWER SOURCE INVENTORS WILLIAM R. STURROCK ALLEN J. BAKER HENRY W. MC ROBBIE ATTORNEY United States Patent 3,361,862 SYSTEM FGR SUPPLYING POWER TO AN ELEC- TRIC ARC METALLURGICAL FURNACE William R. Sturroek, North Tonawanda, Allen J. Baker, Tonawanda, and Henry W. Mcliobbie, Buffalo, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed June 15, I964, Ser. No. 375,139 7 Claims. (Cl. 13--I) This invention relates to electric arc metallurgical furnaces and more particularly to novel systems for providing electrical power to such furnaces.

Electric furnaces have long been used in practicing etallurgical processes. Presently, there has been developed a new electric furnace process for treating metals wherein a directionally stable arc is used as the heat source in the furnace.

In this process at least one directionally stable transferred electric arc column is established in a furnace containing conductive charge material. Power dissipation in such electric arcs is virtually constant and controllable. Arc length, of a directionally stable are as a variable in determining the voltage drop across the arc, can be maintained constant or it can be smoothly varied at will to meet the particular power input requirement in a given process commensurate with maximum dissipation of heat energy in the are for a given power supply.

The power input can be controlled and optimized to meet the requirements of a particular process, that is the melting rate may be increased as required in a particular process by increasing the voltage drop across the are column. This is in sharp contrast to standard furnace practice wherein power input is in essence not controlled smoothly by affirmatively adjusting arc length responsive to the desired power input level but rather it is sought to suppress the erratic changes in the length of a relatively short are.

Generally, the directionally stable transferred DC. are column is established between a non-consumable electrode, for example tungsten, and a water-cooled bottom electrode positioned beneath the conductive charge material with the electron current path being between the non-consumable electrode through the charge material to the bottom electrode. Usually the device having the nonconsumable electrode has a water-cooled copper nozzle provided with a central gas passage through which the current carying arc and are effluent passes on its way to the charge material.

We have discovered a new improved system for supplying power to the electric arc furnace of the type just described. Our invention is predicated on the discovery that stable arc operation can be achieved when an AC. source is connected to a device for producing a directionally stable arc in such a manner that the water-cooled copper nozzle is a current-carrying element in the main arc circuit and receives only positive half cycle voltages. Electron current then flows in the non-consumable tungsten electrode circuit when the tungsten is negative with respect to the bath and flows in the nozzle circuit only when the nozzle is positive with respect to the bath.

One of the advantages of such a system is that since the tungsten electrode carries .a major part of the current for only half cycles, it can carry a higher current than it would in ordinary DC. operation.

Also, operation of two or more arc devices in this manner would eliminate the need for a bottom electrode in the furnace. This is desirable because there are several disadvantages to a bottom electrode. A water-cooled bottom electrode in a hot metallurgical furnace is a safety hazard. Water-cooled or not there is always a possibility of loss of melt from the furnace into the electrode pit. It is not always possible to insure electrical contact between the scrap and the bottom electrode into the melt may be undesirable from a metallurgical standpoint.

Accordingly, it is a primary object of this invention to provide a novel power supply circuit for directionally stable electric arc furnaces.

Another object is to provide a novel power supply circuit from a polyphase A.C. source to a plurality of directionally stable are devices in an electric arc furnace.

A further object is to provide such a power supply circuit from a polyphase A.C. source to a plurality of directionally stable are devices having a tungsten electrode surrounded at least at its arcing end by a cooled copper nozzle.

These and other objects will become apparent or will be pointed out in the following description where reference will be made to the drawings.

FIGURE 1 being a schematic diagram of a preferred embodiment of the invention;

FIGURE 2 is an alternative form of the invention;

FIGURE 3 is an illustration of a single arc device arrangement; and

FIGURE 4 is an illustration of a two are device arrangement.

Referring now to the drawings and particularly FIG- URE 1 illustrating a preferred embodiment of the invention, a three phase are transformer noted generally at T is preferably delta connected on the primary side 1 to a 4-40 volt three phase line. The secondary side 3 of transformer T is preferably Y connected. Each phase 5, 7 and 9 respectively of the secondary Y 3 is split into two legs noted by the reference characters 11 and 13 for phase 5; 15 and 17 for phase 7; and 19 and 21 for phase 9. Legs 11, 15 and 19 are each connected to a non-consumable electrode 23, 25, 27. Such electrode is preferably made from 2% thoriated tungsten. A rectifying element 29, 31, 33 is connected in each leg 13, 17 and 21 between nozzle 35, 37, 39 and the secondary of transformer T.

As mentioned above in describing the circuit relationship of each element, in the preferred system the secondary side of transformer T is usually Y connected. The Y connection provides a higher voltage than does a delta connected secondary and also allows a center point connection to the bath, if desired or necessary. However, a delta connected secondary is also within the scope of this invention.

A tungsten electrode is preferred because of its high melting point and low voltage requirements at high current densities. Other materials may be selected according to their effect on the metallurgical process being performed in the furnace so long as the material is a good electron emitter. A typical directionally stable arc device containing a tungsten electrode and admirably suited for practicing the present invention is described by R. M Gage in US. Patent 2,806,124 issued Sept. 10, 1957.

The rectifying elements 29, 31 and 33 are preferably silicon diodes. However, germanium or selenium rectifiers are also suitable for practicing the invention.

Having described the preferred circuit embodiment of the invention the operation thereof is as follows:

In three phase operation three torches A, B and C are placed in an appropriate electric furnace *F. In the embodirnent shown, the arc is established between the torches A, B and C and the metallic bath shown symbolicially at M by touching the electrodes 23, 25 and 27 to the bath M. Alternatively, pilot-arc power supply systems can be connected between each of electrodes 23, 25 and 27 and the nozzles 35, 37 and 39 whereby a pilot arc is first established therebetween prior to and as an auxiliary for starting the main arc. After the arc is established the arc length is adjusted to a desired length depending on the process conditions necessary within the open circuit range of the power supply being utilized. The process is continued until the terminal point is reached.

FIGURE 2 is an alternative form of the power supply circuit of the invention. In this illustration parts identical with the parts illustrated in FIGURE I bear identical reference characters differing therefrom merely by a prime. In this embodiment an additional silicon diode 4t), 42 and 44- is connected in legs 11, 15 and 19 of each phase between the non-consumable electrodes 23, 25 and 27' and the secondary 3 of transformer T. The additional diode insures that electron current does not flow through the tungsten electrode on the positive half cycle when current should be flowing through the nozzle element. In all other respects the embodiment shown in FIGURE 2 was identical to that shown in FIGURE 1.

It is possible to operate a single torch, single phase using the concepts of our invention. However, in this embodiment, it is necessary to employ a bottom electrode beneath the surface of the molten bath. In FIGURE 3, the A.C. power source could be a Y connected secondary of a 3-phase arc transformer with the center of the Y connected to the bath M. Alternatively the power source could be a single phase are transformer with one top connected to the work and the other to point 52. A silicon diode is connected to leg 47 between the nozzle 49 and the A.C. source. The other leg 51 of the single phase power is connected to a non-consumable electrode 53. In this embodiment the arc is established by touch starting the electrode 53 to the bath M. Electron current flows from the source to electrode 53 through the arc to the bath M and back to the source. On the reverse cycle the electron current will prefer to travel from the bath M to the nozzle 4-9 through rectifier 45 to the source. Thus the tungsten electrode 53 carries main arc current for only one half cycle.

Two torches E and D may be operated from a single phase are transformer power source as shown in FIG- URE 4. In this embodiment one tap of the secondary of the transformer shown in block form is connected to point '54 at torch D and the other tap is connected to point 56 at torch E. A silicon diode 55 and 57 is connected in the circuit from nozzle h and 61 to the power source. Another circuit to the power source includes tungsten electrodes 63 and 65. The are is established by touching electrodes 63 and 65 to the bath M. On one cycle, electron current passes from the A.C. source to electrode 65 through bath M, to nozzle 59 of torch D through rectifier 55 back to the A.C. source. On the reverse cycle, electron current flows from the source through electrode 63 to the bath M through nozzle 61 of torch E and rectifier 57 to the source.

Typical data obainted from the prototype systems illustrating the feasibility of the inventive concept are summarized in Table I.

TABLE I.-EXAMPLES OF OPERATING DATA Number of Volts, Current, Phases Power, Argon/Torch Torches A.C. A.C. w (c.f.h.)

1 120 75 Single... 9. 0 100 3 (55 525 Three 102.0 25

to the ingot iron. An arc length of two inches was then maintained.

TABLE II Time, Volts, Amps, Argon, Power,

min. A.C. A.C. 0.1111. kw

*Iower otl and tapped.

At the end of the heat about 3% lbs. of metal was tapped. The remaining metal formed a skull around the hearth in the furnace due to excessive heat losses in the small prototype unit. However, the operation of the prototype furnace does illustrate the operability of the novel power supply system.

It is to be understood that the embodiments shown in the drawings are for purposes of illustrating various systems and alternatives of the inventive concept and are not to be considered as exhaustive of the alternatives and modificiations capable of being made within the scope of the invention. For example, the embodiments of FIG- URES 3 and 4 could be further modified to include a rectifier element between the non-consumable electrodes and the power source.

What is claimed is:

1. An electric arc metallurgical furnace containing a charge of raw material electrically conductive at least in the molten state including a devlce for producing a directionally stable are having an electrode and a nozzle surrounding and radially spaced from said electrode so as to provide a passage for are gas to flow between said electrode and nozzle in combination with an electrical circuit comprising an A.C. power source, at least one rectifying element connected between said power source and said nozzle so that said nozzle will collect electrons when said nozzle is electrically positive with respect to said molten bath.

2. The combination of an electric arc metallurgical furnace wherein at least three devices producing a directionally stable are having an electrode and a nozzle surrounding and radially spaced from said electrode so as to provide a passage for are gas to fiow between said electrode and nozzle provide the heat energy to said furnace and an electrical power system comprising a three-phase A.C. power source, each of said three-phases containing a first and second leg, said first leg of each phase being connected to said electrode in one of said devices and said second leg of each phase being connected to said nozzle in one of said devices and at least one rectifying element connected in said second leg of each phase between said nozzle and said A.C. power source.

3. System according to claim 2 wherein said rectifier elements are silicon diodes.

4. An electric arc furnace power system according to claim 2 wherein silicon diodes are in both legs of each phase of said three phase A.C. source between said A.C. source and said electrode and nozzle respectively.

5. An electric arc metallurgical furnace containing a molten bath of electrically conductive material including an arc torch device having a non-consumable tungsten electrode and a water-cooled copper nozzle at least partially surrounding and radially spaced from said tungsten electrode as to provide a passage for arc gas to fiow between said tungsten electrode and copper nozzle incombination with an electrical circuit comprising an A.C. power source including an arc transformer connected in circuit with said are torch, at least one rectifying element connected between said are transformer and said copper nozzle of said are torch so that said copper nozzle will collect electrons when said nozzle is electrically positive with respect to said molten bath.

6. Combination according to claim 5 wherein the primary of said are transformer is delta connected and the secondary of said arc transformer is Y connected.

7. An electric arc metallurgical furnace containing a molten bath of electrically conductive material including at least three are torch devices having a non-consumable tungsten electrode and a water-cooled copper nozzle at least partially surrounding and radially spaced from said tungsten electrode as to provide a passage for are gas to flow between said tungsten electrode and copper nozzle in combination with an electrical circuit comprising at least a three phase AC. power source including an are transformer connected in circuit with said arc torch, at least one rectifying element connected between said are transformer and said copper nozzle of each of said are torches so that said copper nozzles will collect electrons when said nozzles are electrically positive with respect to said molten bath.

References Cited UNITED STATES PATENTS 2,907,807 10/1959 Noesen 13-12 BERNARD A. GILHEANY, Primary Examiner. R. N. ENVALL, Assistant Examiner. 

1. AN ELECTRIC ARC METALLURGICAL FURNACE CONTAINING A CHARGE OF RAW MATERIAL ELECTRICALLY CONDUCTIVE AT LEAST IN THE MOLTEN STATE INCLUDING A DEVICE FOR PRODUCING A DIRECTIONALLY STABLE ARC HAVING AN ELECTRODE AND A NOZZLE SURROUNDING AND RADIALLY SPACED FROM SAID ELECTRODE SO AS TO PROVIDE A PASSAGE FOR ARC GAS TO FLOW BETWEEN SAID ELECTRODE AND NOZZLE IN COMBINATION WITH AN ELECTRICAL CIRCUIT COMPRISING AN A.C. POWER SOURCE, AT LEAST ONE RECTIFYING ELEMENT CONNECTED BETWEEN SAID POWER SOURCE AND SAID NOZZLE SO THAT SAID NOZZLE WILL COLLECT ELECTRONS WHEN SAID NOZZLE IS ELECTRICALLY POSITIVE WITH RESPECT TO SAID MOLTEN BATH. 